A thermal transfer recording medium having a support and a heat softening layer coated on the support is widely used for printing (transferring) with a thermal printer such as a word processor. However, the conventional transferring technique using a thermal printer has fundamental problems such as print quality seriously influenced by the smoothness of a transferee medium (ex. a transfer sheet); and a printing speed slower than that of other transferring techniques.
For improving print quality with a transferee medium of poor surface smoothness, there has been proposed techniques of multi-layered heat softening layer; and techniques of improving the transferability of the heat softening layer by incorporating various additives. For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 235189/1986 discloses a thermal transfer recording medium having an intermediate layer containing a supercooling substance (ex. polyethylene glycol), between a heat softening layer (heat-fusible ink layer) and a support. Since the intermediate layer containing a supercooling substance can maintain the heat softening layer in a fused state for a relatively long time, it can afford high print quality even with a transferee medium of poor surface smoothness so fat as the heat softening layer (heat-fusible ink layer) can remain in a fused state.
However, it was learned that either of the above-mentioned techniques for improving the print quality with a transferee medium of poor surface smoothness serves insufficiently when such a technique is applied to a high speed printer developed in compliance with current consumer's demand, i.e. higher printing speed. On a high speed printer, the heat softening layer is not firmly transferred onto a transferee medium because the heat softening layer remains in the fused state for a short duration, thereby a heat softening layer, even that attains good print quality with a transferee medium of poor surface smoothness at a normal recording speed, affords significantly deteriorated print quality at a higher recording speed.
As a result, there is a growing demand for developing a thermal transfer recording medium capable of recording high quality prints on a transferee medium of poor surface smoothness by using a high speed printer.